LED Thermal Management Case Study – C.T.E. Matched Vapor Chambers
There are a growing number of high power LED applications, such as UV curing devices. These products typically have very high localized heat fluxes, greater than 300 W/cm2, but must maintain a tight temperature range so that the output optical wavelength remains constant. Direct die attachment can lead to mechanical stresses at the interface if the coefficient of thermal expansion (C.T.E.) is mismatched between the die and the substrate. The standard method to overcome this is to add an interface material, such as thermal gap pads or thermal pastes, to accommodate the mismatch. Unfortunately the presence of this thermal interface layer increases the thermal resistance and likewise increases the temperature on the LED device itself.
Vapor Chambers are an important tool in LED thermal management, since they act as flux transformers, spreading the high input heat flux over the entire surface of the vapor chamber. This allows the heat to be removed from the vapor chamber by conventional cooling methods. Most vapor chambers are limited to an input heat flux of about 75 W/cm2, however, ACT has developed a C.T.E matched vapor chamber that allows for direct bonding of the LED; see Figure 1. This unique device has been demonstrated to dissipate heat fluxes as high as 700 W/cm2 and 2kW overall. The evaporator thermal resistance of vapor chambers with this wick design is only 0.05 K-cm2/W.
The overall envelope structure is aluminum nitride with a direct bond copper exterior. The copper on the inside of the vapor chamber ensures that the well-known water/copper performance is maintained. In areas where the heat source is to be attached, the copper layer is removed exposing Aluminum nitride. Aluminum nitride has a CTE of ~5.5 ppm/⁰C, which is close to many common semiconductor materials. The devices can be directly attached to the vapor chamber, eliminating the need for a thermal interface layer.